The present application relates to the magnetic resonance arts. It finds particular application in conjunction with diagnostic imaging at surgical sites and will be described with particular reference thereto. However, it is to be appreciated that the invention will also find application in other magnetic imaging, spectroscopy, and therapy applications.
Early magnetic resonance imaging systems were based on solenoid magnets. That is, a series of annular magnets were placed around a bore through which a magnetic field was generated longitudinally. A patient was selectively moved axially along a horizontal central axis of the bore to be positioned for imaging. Magnetic resonance imaging systems with solenoid magnets tended to be claustrophobic to the patient. Moreover, access to the patient for surgical, minimally invasive procedures, physiological tests, equipment, and the like was limited and awkward.
To provide for patient access and reduce the claustrophobic effect in patients, open or vertical field magnets have been devised. Open magnets typically include a ferrous flux return path in the form of a "C", "H", or four-poster arrangement. The flux return paths have an open gap within which the patient is disposed for imaging. Due to the difference in the susceptibility of the flux return path and the air in the patient gap, there tends to be non-linearity and other magnetic flux errors in the patient receiving gap. In order to generate a more uniform magnetic flux field through the gap, a large ferrous pole piece is typically positioned at the ends of the flux return path on either side of the patient receiving gap. The pole pieces are shaped and contoured, as appropriate, to generate a more uniform magnetic flux between the pole pieces. Typically, a heavy ferrous ring, known as a Rose ring, is positioned along the circumference of the pole piece to drive the magnetic flux towards the center of the pole piece and the patient receiving gap.
Although the use of pole pieces has been successful, there are drawbacks. First, in magnetic resonance imaging, magnetic field gradients are generated across the imaging volume. The gradient coils are positioned between the pole pieces and the patient. When the gradient coils include shield coils, the space occupied by the self-shielded gradient coils is even larger. The physical space occupied by the gradient coils exasperates the tradeoff between the desire to have a large patient receiving gap for better patient access and less claustrophobia, and the desire to position the pole pieces closer together for a more uniform magnetic field. Second, the pole pieces are typically thick ferrous disks with a diameter about 2-3 times the height of the patient receiving gap. The massive metal pole pieces raise difficult engineering design problems to provide for their stable support with a minimal blocking of patient access.
This application provides a new and improved magnetic resonance imaging system which overcomes the above-referenced problems and others.